Viruses are major causes of disease and death throughout the world. Although vaccines and public health measures have greatly reduced the incidence of certain viral infections, such approaches have been less successful in tackling many viruses of significant medical and/or veterinary importance. Even if a generally protective vaccine exists, it is challenging to achieve vaccination of all individuals. Furthermore, obstacles to effective immunization can arise due to factors such as immune senescence and treatment with immunosuppressive medications. Pharmacological therapies have been developed against some viruses, with human immunodeficiency virus (HIV) being a notable example. However, there are still relatively few viral diseases for which effective drugs are available. There is a need for new antiviral compounds and for new approaches to identifying such compounds.